if you rebuilt them, and you know you did that right.
and you have a new master on there that you know is working properly,
and you've not been able to find any leaks anywhere,
then more likely than not, you just have air in the lines.
When the calipers are off of the rotors, and you push the brake pedal, do the pistons come out properly?
Can you pump them out a little bit that way, and then squeeze them back in?
Does anything change when you do that a few times?
Are you sure that the front calipers are not functioning and it isn't something dumb like the pads are crap? They could have soaked up some oil, or even been glazed over and just not producing any stopping power. (it can happen, but usually you get some kind of sequel, whine, some kind of noise.